In the case of fast-neutron nuclear reactors, the primary fluid for cooling the reactor generally consists of liquid sodium filling a stainless steel vessel of large dimensions closed by a very thick horizontal slab.
When the reactor is stopped after a period of operation, it is necessary to continue the cooling of the core assemblies, because some residual radioactivity remains, generating heat in this core.
In high-power nuclear reactors, the quantity of heat to be removed is large and the principal heat exchange circuit of the reactor is generally used to carry out its cooling after stopping. In the case of integrated-type reactors, this circuit incorporates intermediate sodium-sodium exchangers and pumps for circulating the primary sodium. These pumps operate at a low speed during the cooling after stopping.
However, if a technical incident causes a stoppage of the normal operation of the principal cooling circuit, the core can no longer be adequately cooled. Excessive heating of the core can result in very severe accidents, so that an emergency cooling circuit, completely separate from the principal circuit, of great simplicity and high reliability, is provided.
Such an emergency circuit incorporates a sodium-sodium heat exchanger partly immersed in the primary reactor fluid. This heat exchanger incorporates a bundle of tubes inside which circulates secondary sodium which heats up in contact with the primary sodium present in the reactor vessel. The secondary sodium circulated through the bundle is itself cooled outside the reactor vessel, in a sodium-air exchanger.
In the case of fast-neutron reactors of high power, for example 1,500 or 1,800 Mwe, it is necessary to employ several sodium-sodium emergency exchangers immersed in the reactor vessel. It is necessary to restrict the number of these sodium-sodium emergency exchangers for cost reasons, and to reduce the number of passages in the reactor slab. The sodium-sodium emergency exchangers must therefore be relatively large.
Furthermore, these heat exchangers are subjected to very high thermal stresses, with the result that their design presents technical problems which are difficult to solve.
In the majority of cases, the sodium-sodium emergency exchangers are of the type with bundles of hairpin tubes immersed directly in the primary sodium. These tubes are placed inside an outer shell which is open at its base and perforated over a large part of its side surface.
The U-tubes are joined at one of their ends to a first tube plate, and at their other end to a second tube plate offset relative to the first along the height of the heat exchanger. These tube plates enable the secondary sodium in the tubes to be sent to the central part of the exchanger and to be recovered at its peripheral part. The cooled sodium descends in the tube branches situated at the central part of the exchanger and rises in the tube branches situated at the periphery of the latter. While travelling in the tubes, the secondary liquid sodium heats up in thermal contact with the primary sodium through the tube walls. This results in very large temperature differences between the various parts of the exchanger. The latter can also be subjected to large temperature changes over time. This results in thermal stresses which can be very high in some parts of the exchanger, and it is necessary to design exchangers of such structure that it enables these thermal stresses to be reduced to acceptable levels.
Furthermore, the tubes forming the exchange bundle must be efficiency braced to prevent their relative movement under the effects of heat and vibrations. The gives rise to problems in the assembly of the heat exchanger which are difficult to resolve.